Paired with Shona Morrison and trailing by two shots on the final end, Barclay delivered the last bowl of the final right on the money to secure two shots and grab victory from the jaws of defeat.
The Andersons Bay pair had seemingly secured the title after lead Williams secured three shots around the jack.
Penty was unable to counter Barclay's pinpoint accuracy in opening up the front of the head with her first two bowls. Then, in a rare error, Penty's third and final bowl landed short and wide.
The target for Barclay was wide open and she drew two shots to win the title 18-17.
Penty and Williams got away to a flying start in the opening rounds and it was not until the seventh end that the Taieri pair got ahead to hold a 7-6 lead.
Three shots on the 10th end and another three shots on the 13th resulted in Taieri holding a 15-8 lead with five ends to play and having one hand on the trophy.
But Andersons Bay bounced back. They scored two shots on the 14th end, three on the 15th and a further three on the 16th to lock the game up at 16-16 with two ends to play.
Penty and Williams then claimed a further shot on the 17th end to hold a 17-16 advantage going into the final end.
But it was not to be as the Taieri duo triumphed.
"They fought back really well to have the game locked up at 16-16 on the 16th," Barclay said.
"One down and one to go they put a couple of really good shots on the head to put the pressure on us. I was struggling with the wide hand, so I went for the narrow hand, thinking the bowl would hold if I got a direct line, which it did. The last one was a cracker."
Morrison only played as Lorraine Turnbull was ill.
- The final of the men's champion of champion pairs between Doug Thomas and Dave Natta (Fairfield) and Steve Fleming and Steve Wilson (Forbury Park), will be played at 4.30pm on Thursday at the Taieri Bowling Club.
